Method of preparing a sensitized thermographic copy sheet and resultant sheet



Nov. 12, 1957 w. E. GORDON ETAL 2,813,042 METHOD OF PREPARING A SENSITIZED THERMOGRAPHIC COPY SHEET AND RESULTANT SHEET Filed 001:. 2, 1952 PAPE R IMPREGNATED WITH T HIOUREA POLYVALENT METAL SALT, UREA IMPREGNATED WITH THIOUREA POLYVALENT METAL SALT, UREA |MAGE= THERMAL REACTIGN PRODUCT OF THIOUREA POLYVALENT METAL SALT, UREA mv-ENToRs.-. WILLIAME. GORDON SAMUEL E. EATON BY W ha .Serial No. 178,226, filed on United States Patent METHOD OF PREPARIWG A SENSITIZED THER- MOGRAPHIC COPY SHEET AND RESULTANT SHEET William E. Gordon, Sharon, and Samuel E. Eaton, Hingham, Mass, assiguors to A. B. Dick Company, Niles, 12L, a corporation of Illinois Application October 2, 1952, Serial No. 312,866 13 Claims. (Cl. 117-36) This invention relates to a system for producing copy and to articles of manufacture for use in same. It relates more specifically to a new and improved sensitized sheet used in the production of copy directly from an original by what will hereinafter be referred to as thermographic technique.

In the copending application of Allan I. Roshkind,

August 11, 1950, now abandoned, and entitled Method for Preparing Copy, description is made of a new and improved thermographic copying process wherein a sheet of paper or the like sensitized throughout to produce a color change in areas raised to elevated temperature is positioned in surface contact with an original having an image formed thereon of visible and/ or infra red ray absorbing material. When radiations rich in infra red are directed onto the original in sufficient intensity, the radiations are absorbed by the material in the image and converted into heat to form a corresponding heat pattern which may have a temperature Within the range of 250-1000" F. or more. The heat pattern transfers to the sensitized copy sheet in sur face contact therewith and causes the image to be reproduced therein by thermal reaction.

In the aforementioned application of Allan I. Roshkind, the copy sheet is sensitized with acids or strong acidic salts which, at the elevated temperature provided in the heat pattern, causes a reaction to take place chiefly by way of degradation of the fibers of the base sheet to develop the image. Such degradation reaction of the type described provides colors which are limited in character to browns or various deep shades of brown. Furthermore, the acidic substances appear to be able slowly to react With the fibers of the base sheet under normal atmospheric conditions whereby background colors are developed and the base sheet becomes embrittled mate rially to decrease the durability and flexibility of the copy sheet.

Black copy of fairly good quality is capable of being produced thermographically by sheets sensitized with a composition based upon thiourea or a water soluble monoalkyl derivative thereof such as methyl thiourea, ethyl thiourea, allyl thiourea, propyl thiourea and the like, and nickel acetate or other bivalent metal salts of low aliphatic acids, such as the acetates, formates, propionates, lactates and the like, such as described in the copending applications of Clarence J. Shoemaker, Ser. No. 280,161, filed on April 2, 1952, now abandoned, entitled Thermographic Copying Process and Elements for use in Same, and Ser. No. 286,027, filed on May 3, 1952, now abandoned, entitled Method and Materials for Producing Copy. It has been found in systems of the type described that the thermal reaction may be catalyzed by the addition of alkali metal salts of low aliphatic acids such as by the addition of sodium acetate, ammonium acetate, potassium formate and the like present in an amount ranging from equal parts by weight to three times the weight of thiourea and metal salt, and that further improvernent in the avoidance of the development of back- 2,813,042 Patented Nov. 12, 1957 ground colors, improved stability and catalysis of the color forming reaction may be secured by the still further introduction of small amounts of inorganic polyvalent metal salts, such as magnesium chloride or sulphate, zinc chloride, aluminum sulphate and the like and present in an amount up to 20 percent by weight of the thiourea and metal salt.

It is an object of this invention to provide a still further improvement in a sensitized copy sheet for thermographic duplication and it is a related object to produce a sensitized copy sheet of the type described capable of producing copy of excellent quality under conditions which do not harmfully affect the copy sheet or the permissible use thereof.

Another object of this invention is to provide a sensitizing composition which is low in cost and easy to manufacture and apply in the production of a sensitized copy sheet characterized by excellent shelf life, good stability, high sensitivity at low temperature levels and capable of producing copy of good color intensity, sharp detail in the letter outlines, with minimum burn-out and spread of letter outlines, and with little, if any, effect on the original so as to permit repeated use thereof as a master in the production of additional copies.

A further object is to produce copy sheets sensitized with a combination of materials for the production of black copy without noticeable attack of the base sheet and with minimum harm to the original from which it is formed and it is a related object to provide a new and improved method for thermographic duplication.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of this invention is shown in the accompanying drawing, in which Figure 1 is a perspective view of a sensitized sheet embodying the features of this invention, and

Figure 2 is an imaged sheet prepared in accordance with the practice of this invention.

It has been found, in accordance with the practice of this invention, that the thiourea-organo metallic salt system previously described as forming the base of a composition for sentitizing base sheets in the production of copy by thermographic duplication is greatly improved by the incorporation of substantial amounts of urea. When incorporated in desired concentrations, the presence of such other materials as alkali metal salts of low aliphatic acid in an amount ranging from equal parts by weight to three times by weight of the thiourea and metal salt and polyvalent or bivalent metal sulphates and chlorides in an amount up to 20 percent by weight of the thiourea and metal salt and which have previously been used to catalyze the thermal reaction and stabilize the some instances such additional components have been found undesirable.

The following will illustrate a sensitizing composition embodying features of this invention for use in the treatment of base stock to produce sensitized sheets for thermographic duplication:

Example 1 Percent Urea 30 Nickel acetate l0 Thiourea 3 Water 57 a eoea sufficiently high concentration of the color forming components may be secured to produce copy of sufficient intensity with less material. This also reduces the possibility of the copy sheet sticking to the original or transfer of material reduced to flowable condition in the sensitizing composition to the original, especially when the sensitized copy sheet is positioned with the uncoated side in surface contact with the original. The sensitizing composition may be allowed to dry following application onto the base stock at room conditions but drying may be accelerated by circulation of air over the treated base stock and still further speed in the drying operation may be secured by the use of an air drying system heated to elevated temperature but below a temperature which will cause the color forming reaction to take place; For example, the coated or impregnated base stock may be dried at a temperature below 125 F.

Suitable base stock for sensitizing with compositions of the type described include tissue formed of cellulosic or other natural fibers, alone or in various combinations.

Suitable tissue may be formed of natural fibers and synthetic fibers of the type nylon, Saran, Vinyon and the like, or in combination with mineral fibers such as glass fibers, asbestos fibers, rock wool fibers and the For impregnation to sensitize the sheet throughout, use is preferably made of a porous tissue of the type known as Onota tissue and for coating, use may be made of paper stock of the type Washington Linen and the like. Instead of fibrous tissue, the treating composition in dilute solution may be applied onto the base sheet in the form of continuous films r layers such as may be formed of highly beaten or partly gelatinized pulp fibers, as in glassine paper, or plastic film stock or laminates formed with paper or the like.

it is preferred to maintain about a stoichiometric relationship between thiourea and nickel acetate corresponding to a ratio of about 1 part by weight thiourea to about 3.28 parts by weight nickel acetate but acceptable copy is capable of being produced when the ratio is maintained within the range of 1 part by weight thiourea to /25 parts by weight nickel acetate. The higher ratios of 1 part thiourea to 3-5 parts nickel acetate are preferred when applying a light coating to sensitize one side of the sheet stock because then the higher concentration of nickel acetate and urea will have little effect on sticking or contamination of the original while copy of sufficient intensity will be able to be produced in fractions of a second with the temperatures made instantaneously available in the thermographic radiation process.

The amount of urea must be balanced between the effect on temperature stabilization which appears to result from its use and its effect in slowing the color forming reaction between the base substances under the temperature conditions existing. In other words, it is undesirable to use somuch urea as will cause the reaction to be slowed to the extent that copy of readable quality cannot be produced, yet it is necessary to use an amount of urea sutficient for stabilization of temperature and to prevent the spread thereof so as to produce copy of superior quality. Important also is the effect which the urea has on the sticking characteristics of the sensitized copy sheet when present in high concentration resulting from its melting under conditions of use coupled with the possible formation of a glassy phase by reaction in combination with nickel acetate which may introduce sticking in transfer to the original while in the molten state and impair the further use thereof in the production of copies. In practice, excellent results have been secured when the amount of urea is equal to the total of thiourea and metal salts but it is preferred to make use of urea in proportions 2 to times greater than the amount of thiourea and metal salts.

While it is preferred to use nickel acetate or other water soluble salts of nickel such as the nickel salts of low aliphatic acids of the type formic, acetic, propionic, and

lactic acids or the like, or of inorganic acids such as the nickel chlorides, nickel sulphates and the like, use may also be made of inorganic or of low aliphatic salts of other bivalent metals, such as of lead, cobalt, copper, iron and the like, or of polyvalent metals such as antimony sulphate, antimony acetate, antimony chloride and the like. Such metallic components become free to react with the thiourea in combination with urea under conditions of use to develop copy by chemical reaction between the various components. Although metallic substances such as copper and iron and the like bivalent and polyvalent metal salts may be used, it is preferred to make use of metallic components incapable of reduction since, under the conditions of use, thiourea appears to have reducing characteristics.

Although copy of good quality can be produced from a system wherein the copy sheet is sensitized only with thiourea or nickel acetate and the like, vast improvement in copy quality and sharpness of copy has been found to result from the addition of prescribed amounts of urea. While the urea may not enter directly into the color forming reaction between thiourea and nickel acetate, it obviously has a pronounced elfect upon the course of the reaction and the conditions under which it takes place and favorably influences the conditions existing to produce copy of good quality.

Various theoretical considerations have been advanced of the effect of urea on the color forming thermal reaction, all or some of which may actually exist. It has been determined, for example, by difierential thermal analysis that urea provides an endothermic peak at its melting point of about 132 C. followed by a general rising endothermy to a very sharp exothermic peak at about 2002l0 C. where endothermicdecomposition is believed to take place to form ammonia and cyanuric acid or biuret which crystallizes out as a residue. Other peaks occur later but no explanation is yet available of the reactions which take place or the reasons therefor.

Since urea melts at 132 C., which is believed to be slightly below the temperature required to enhance the color-forming reaction of the sensitizing materials, it will be apparent that in the heated areas corresponding to the heat pattern formed in the original, the urea must be reduced to molten condition. This prevents rapid spreading of the heat pattern laterally in the copy sheets until the urea in adjacent areas is also reduced to molten condition. When present in substantial amounts in the sensitizing sheet, this requirement for progressive melting prevents the temperature from rising in areas adjacent that conforming to the generated heat pattern and therefore materially reduces the spread and the production of ragged copy which has been somewhat characteristic of prior copy systems.

Another possible explanation of the desirable effect derived from the use of urea, in substantial amounts as an ingredient in the sensitizing composition resides in the stabilization of temperature around ZOO-210 C. which corresponds to the endothermic decomposition range of urea and to the temperature range which is believed available in the heat pattern generated by radiation of the original in the described thermographic process. Stabilization of existing temperatures at or about ZOO-210 C. provides a desirable control of the reaction rate and apparently markedly reduces autocatalytic effects with the result that sharp copy of good quality is produced. These advantages are not available in systems based on the use of thiourea and bivalent metal salts of inorganic or organic acids.

A third possibility which may take place in combination. with the theoretical considerations previously discussed to enhance. control of the color forming reaction in the. productionv of copy resides in the conversion of urea under reaction conditions to what appears to be white opaque crystals vof cyanuric acid and/or biuret and the deposition of such crystals in a manner to gray down the copy and reduce self-absorption and autocatalysis. The deposition of white crystals in areas immediately adjacent the heat pattern tends to accent the latter formation and improves the clarity and detail in the copy and the apparent intensity thereof.

Generally it is believed urea is capable of functioning as an excellent solvent available at the high temperature of reaction to combine the ingredients and that when present in large amounts has a sufiicient diluting effect on the reactants to control their reaction rate for the production of good quality copy in reasonable time consistent with the image spread and time cycles imposed by existing equipment.

Theoretically, desulphurization of the thiourea molecule is favored under high pH conditions and it is believed therefore to be enhanced by the evolution of ammonia from urea when heated to high temperature coincident with the conversion thereof to cyanuric acid and/or biuret. These conditions providing for a rise in pH during exposure to the heat pattern tend further to accelerate the color-forming reaction at lower temperature and in less time.

There is considerable experimental evidence that at elevated temperatures the sensitizer reaction is inhibited by urea. This may result either from the production of end-products in the decomposition of urea which are acidic and would therefore tend to depress the desulfurization reaction, or from the formation of a complex of urea or its decomposition products with the metallic component thereby lowering the availability of the latter for the sensitizer reaction. This inhibiting effect may become operative at a temperature somewhat above that required for the sensitizer reaction so that portions of the heat pattern which are overheated do not produce excessive intensity of color in the copy at those points. Whatever the reason, the effect of urea is to make the copy uniform in color intensity and with little lateral spread, thereby providing for faithful reproduction of letter outlines Without closing of loop letters which is somewhat characteristic of earlier systems.

Additional sensitizing compositions which may be used to illustrate the practice of this invention are as follows:

Example 2 Grams Nickel acetate 3-6 Thiourea 1V2-6 Urea 20-60 Example 3 Nickel chloride 6 Thiourea 4 Urea 30 Example 4 Urea 30 Nickel chloride Ethyl thiourea 4 Example 5 Urea 40 Cobalt chloride 10 Allyl thiourea 4 Example 6 Nickel acetate 6 Thiourea 6 Urea 42 Magnesium chloride /2 Example 7 Nickel chloride 6 Thiourea 4 Urea 40 Sodium formate 20 The compositions described above may be dissolved to form solutions having concentrations ranging from 10-50 percent by weight for application as a coating or impregnating composition onto sheet stock.

In use, the sensitized copy sheet is positioned preferably with the uncoated side, in the event that the sensitizing composition has been applied as a coating to the base sheet, in surface contact with the original in which the letter outlines are formed with material capable of absorption of infra red ray or other heat gen erating rays. When the desired contacting relation has been established, radiations are directed onto the original and absorbed in material forming the letter outlines and converted into heat to form a heat pattern. The radiations striking adjacent areas of the original are substantially reflected or otherwise dissipated so that little, if any, heat is formed. When the radiations directed onto the original are of suflicient intensity to form a heat pattern having a temperature in excess of about 350 F., the heat which transfers to the copy sheet in surface contact therewith causes a thermal reaction to take place in a short period of time with the formation of black copy in corresponding areas of the copy sheet.

This immediate reproduction of copy from the Original permits the immediate detection of errors in the copy sheet so that subsequent reproductions can be made to produce improved copy or to correct errors inadvertently made. The thermal reaction to produce color as a reaction product between the sensitizing materials has little, if any, effect upon the characteristics of the base sheet to permit normal use of the copy in the various processes for which it was intended and in the further reproduction of copies therefrom.

While description has been made of the use of nickel, zinc, cobalt, copper, aluminum and antimony as the metallic components in various salts, it is intended to include any similar metallic salt capable of forming a metal sulphide or other colored reaction product with thiourea under conditions of use in the presence of large concentrations of urea.

Numerous changes may be made with respect to the ingredients and the proportions within the limitations defined and in the manner of application and use in a duplicating system of the type described without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. An article of manufacture of the type described comprising a non-conductive, relatively non-infra red absorbing base sheet sensitized with the combination of thiourea, a salt of a polyvalent metal capable of reaction at elevated temperature to form a colored reaction product with thiourea and urea present in amounts ranging from equal parts by weight to ten times the Weight of the thiourea and polyvalent metal salt.

2. An article of manufacture of the type described comprising a non-conductive, relatively non-infra red absorbing base sheet sensitized with the combination of urea, thiourea, and a salt of a polyvalent metal capable of reaction at elevated temperature to form a colored reaction with the thiourea in the presence of urea and present in the ratio of one part by weight thiourea to l to 5 parts by weight of the metal salt and in which the urea is present in amounts ranging from equal parts by weight to ten times the weight of the thiourea and polyvalent metal salt.

3. An article of manufacture of the type described comprising a non-conductive, relatively non-infra red absorbing base sheet sensitized with the combination of thiourea, a salt of a polyvalent metal capable of re action at elevated temperature to form a colored reaction with thiourea at elevated temperature and present in the ratio of one part by weight thiourea to l to 5 parts by weight of the metal salt and urea present in amounts rangingfrom equal parts by weight to ten times the Weight of the thiourea and polyvalent metal salt.

4. A sensitizedcopy sheet for thermographic duplication directly from an original comprising a non-conductive, relatively non-infrared absorbing base sheet and a composition sensitizing the base sheet comprising thiourea, a salt of a polyvalent metal capable of reaction at elevated temperature to form a colored reaction product with thiourea at elevated temperature and present in the ratio of one part thiourea to 1 mi parts by weight of the metal salt and urea present in amounts ranging from 2 to 10 times the totalweight of thiourea and metal salt.

5. The method of preparing a sensitized sheet for the production of copy by thermographic technique comprising treating a non-conductive, relatively non-infra red absorbing base sheet with acomposition containing thiourea, a salt of a polyvalent metal capable of reaction at elevated temperature to form a colored reaction product with thiourea in the presence of urea and present inthe ratio ofone part by weight thiourea to 1 to 5 parts by Weight metal salt', and urea present in amounts ranging from equal parts to ten times the Weight of the total of thiourea and metal salt.

6. An article of' manufacture of the type described comprising a non-conductive, relatively non-infra red absorbing base sheet sensitized with the combination comprising: (1) a thio compound selected from the group consisting of thiourea and a water soluble monoall-zyl derivative of thiourea; (2) a salt of a polyvalent capable of reaction at elevated temperatures to form a colored reaction product with thiourea, and (3) urea, in which the materials are present in the ratio of 1-10 parts by weight of the urea to 1 partby weight of the thio compoundand metal salt and 1 part by weight of the thio compound to /25 parts by weight of the metal salt.

7. An article of manufacture as claimed in claim 6 in which the sensitizing composition contains in addition an alkali metal salt of a low aliphatic acid present in the ratio of 1-3 partsby weight of the alkali metal salt to 1 part by weight of the thio compound and metal salt.

8. An article of manufacture as claimed inclaim 6 in which the sensitizing composition contains in addition a bivalent metal salt of an inorganic acid present in amountszupr' to 20: percent: by weight of'the thio. com pound: and metal salt;

9. An article. of. manufacture as claimed in claim 6 in which; the sensitizing composition contains in addition a salt of a bivalent metal incapable'of forming a colored reaction product and present in amounts up to 20 percent by weight of the thio compound and metal salt.

10. Thexmethod of preparing a sensitized sheet for the production of'copy by thermographic technique comprising treating a non-conductive, relatively non-infra red absorbing base sheet with a composition containing; (1) a thio compound selected from the group consisting of thiourea and a' water soluble monoalkyl derivative of thiourea; (2) a salt of a polyvalent metal capable ofreaction at elevated' temperatures to form a colored reaction product with thiourea; and (3) urea, in which the materials are presentin the ratio of; l-10 parts by weight of the urea to 1'- part by weight of the thio-compound andmetal; salt and'lpart by weight of the thio compound to /2--5 parts by weight of the metal salt;

11. The method as claimed in claim 10 in which the sensitizing composition contains in addition an alkali metal salt of a low aliphatic acid present in the ratio of 1-3parts by weight of the alkali metal salt to 1 part by weight of the thiocompound and metal'salt.

12. The method as claimed in claim 10 in whichv the sensitizing composition contains in addition a bivalent metal. salt of an inorganic acid present in amounts up to 20 percent by Weight of the thio compound and metal salt.

13. The method as claimed. in claim 10 in which the sensitizing composition; contains in addition a salt of a bivalent metal incapable of-forming a colored reaction product and present in amounts upto-20percent byweight of the thio compound and metal salt;

References Cited inthe file of this patent UNITED STATES PATENTS 1,844,199- Bicknell Feb. 9, 1932 1,968,037 Hartman July'31, 1934 2,503,758 Murray Apr. 11, 1950 2,625,494 Morrison Jan. 13, 1952 2,663,654 Miller et a1 Dec. 22, 19 53 FOREIGN PATENTS 8,819 Australia Aug. 12, 1927 

1. AN ARTICLE OF MANUFACTURE OF THE TYPE DESCRIBED COMPRISING A NON-CONDUCTIVE, RELATIVELY NON-INFRA RED ABSORBING BASE SHEET SENSITIZED WITH THE COMBINATION OF THIOUREA, A SALT OF A POLYVALENT METAL CAPABLE OF REACTION AT ELEVATED TEMPERATURE TO FORM A COLORED REACTION PRODUCT WITH THIOUREA AND UREA PRESENT IN AMOUNT RANGING FROM EQUAL PARTS BY WEIGHT TO TEN TIMES THE WEIGHT OF THE THIOUREA AND POLYVALENT METAL SALT. 